Specification of the cardiac lineage and expression of cardiac specified genes may require combinatorial interactions between transcription found to enriched during the emergence of cardiac progenitor cells. Serum response factor (SRF) may play a leading role in the commitment of cardiac progenitors The recent homologous recombinant knockout of the murine SRF gene locus supports the observation that SRF is required for the appearance of cardiac mesoderm during mouse embryogenesis. However, because SRF null mutants cause such an early embryonic lethality we do not know how SRF actually contributes to early heart muscle commitment and differentiation. We have strong evidence that SRF acts as a myogenic restricted platform to interact with other regulatory proteins and ultimately alter the regulation of specific gene programs. Our studies suggest that SRF facilities binding of the murine homeobox transcription factor, Nkx2-5 and GATA4 to serum response elements resulting in the activation of the endogenous alpha-cardiac actin gene; provides compelling evidence that SRF, Nkx2-5 and GATA-4 are mutually interactive of the endogenous alpha-cardiac actin gene; provides compelling evidence that SRF, Nkx2-5 and GAT-4 are mutually interactive and instrumental for cardiac gene expression. In the proposed studies, we will determine how the SRF gene is regulated during the appearance of cardiac gene expression. In the proposed studies, we will determine how the SRF gene is regulated during the appearance of cardiac mesoderm and the elaboration and formation of the embryonic mouse heart. The Specific Aims of the proposal are: Aim I: How does the murine genomic loci encoding the SRF gene allow for the high level expression during cardiogenesis? Hypothesis 1: Growth factors such as BMP2/4 and FGF4 signals activate SRF gene expression in the cardiac crescent. Overlapping transcription factors Nkx2-5, Smads, GATA-4 and -5, and TBX2,3 and 5 expressed in the cardiac field, induced by these growth factor morphogens, are involved with activating and or repressing SRF expression, SRF_5 an alternative sliced SRF acts as a dominant negative factor that blocks cardiogenesis. These transcription factors activate cardiac specific expression SRF have a direct role in specifying early cardiac mesoderm? Hypothesis 2: The conversion of mesoderm to cardiogenic lineages to drive the cardiac program in the intact mouse embryo requires SRF gene activity. SRF has a role in L/R asymmetry and the conversion of neural crest cells into aortic arch progenitors Aim III: Does SRF require direct association with Nkx2-5 and GATA4 to activate the cardiogenic program? Hypothesis 3: The MADS Box serves as a master regulatory platform that allows for switching of genetic programs depending upon specific factor-factor associations with either of its MADS Box alphaI and or alphaII coils. Combinatorial interactions shared between SRF with Nkx2-5 and GATA4 is obligatory for the progression of mesoderm precursor cells to committed cardiac mesoderm and requires specific physical association through the alphaI coil of the MADS box.